As Earth's climate has warmed, one group of species that has not fared well has been corals, the sedentary marine species which lives symbiotically with algae. Warmer waters cause the algae to become heat-stressed, causing them to die or be expelled by the coral. This causes coral bleaching, a fatal phenomenon that has occurred worldwide with increasing frequency. A team of researchers from the ARC Centre of Excellence for Coral Reef Studies and James Cook University have now revealed the complex molecular signals that lead to the coral's self-inflicted death.

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The team conducted experiments with Acropora corals from the reef at Heron Island which is along Australia's Great Barrier Reef. They found that the cascade of molecular signals commences when ocean temperatures reach a level which is 3 degrees lower than those associated with coral bleaching. Therefore, the cascade of events leading to bleaching come before the actual bleaching occurs. The corals have a premonition of warmer waters in their future.

The cascade of events eventually leads to apoptosis, or programmed cell-death. This process is common with living organisms which will deliberately destroy weak or infected body cells, effectively amputating the cell which has become a liability to the body.

"Our results suggest that the control of apoptosis is highly complex in the coral-algae symbiosis and that apoptotic cell death cascades potentially play key roles in tipping the cellular life or death balance during environmental stress prior to the onset of coral bleaching," explains lead author Dr Tracy Ainsworth. "It is also clear that this chain reaction responds significantly to subtle, daily changes in the environment and to sea temperatures which were generally thought till now to have little impact on the function of coral and its symbiotic algae."

The researchers also found that while the cascade of events kill off some cells, it actually bolsters the strength of other cells. This is done in anticipation for when the warmer waters have moved off, allowing the corals the possibility of a speedy recovery.

"The next step in our research will be to see how we can use this new insight into the processes of coral bleaching to understand their recovery mechanisms," said Dr. Ainsworth. "We also need to know more about how this process works at lower temperatures, or under varying temperatures. That in turn will lead us to explore ways that coral reef managers and users can perhaps minimise other stresses on the reef in order to give it the best possible chance of recovery from bleaching."

The study has been published in the journal, Scientific Reports, which is published by Nature.